Thin films of materials with appropriate thickness and refractive index have important applications in fields such as semiconductors and optics. If a thin film is deposited on a lens, for example, the reflection of particular wavelengths of light from the surface of the lens can be almost completely suppressed. Because of the beneficial effects of thin films, such as antireflection, substantially all high quality optical components are provided with optical films.
Thin films generally include multiple layer films as well as films having a refractive index with a continuous gradient. In multiple layer optical films, such as quarterwave stacks, at least two different materials, one with a relatively high index of refraction and a second with a relatively low index of refraction, for example, are typically deposited in a controlled sequence of alternating layers of specified thicknesses to obtain the desired (specified) optical characteristics. Ideally, the deposition process is controlled by monitoring the thickness of each layer as it is deposited and terminating deposition when the layer reaches the desired thickness. The sought after ability to deposit each layer to precisely the correct thickness would provide flexibility in designing a wide range of multiple layer coatings for various transmission and reflection spectra, for example. Antireflection coatings, laser dielectric mirrors, television camera edge filters, optical bandpass filters, and band-rejection filters are some examples of the many useful devices that include multilayer thin film coatings.
Background information on fabrication of thin films is presented in Macleod, Thin-Film Optical Filters, 2nd Ed., pp. 423-445, MACMILLAN (New York, 1986). During fabrication of thin films, "accuracy" describes the thickness error of any layer from its design specification, and "stability" describes the nature of error accumulation as deposition of multiple layers proceeds. Conventional methods of monitoring and controlling the deposition of thin films include use of a quartz crystal monitor or a single wavelength optical monitor. A quartz crystal monitor, which measures accumulated mass, has fair accuracy but poor stability. A single wavelength optical monitor has poor accuracy because of the difficulty in determining the precise reflection turning point that establishes a quarterwave thickness, but has good stability for quarterwave stacks and single cavity bandpass filters. Single wavelength optical monitors require frequent monitoring chip changes because of effects such as reflectance saturation and reflection changes when producing double cavity bandpass filters. When switching monitoring chips, the new chip may not be the same temperature and may have a different sticking coefficient, which can produce errors in the deposited layers.
A broadband spectral monitor has been used for the deposition of gradient index optical films, such as rugate filters, as described in U.S. Pat. No. 5,000,575 issued to Southwell et al., the teachings of which are incorporated herein by reference. Rugate filters comprise single layer thin films having small, continuous refractive index modulations around an average refractive index value. A broadband spectral monitor works well for such gradient refractive index films because it provides a measure of total optical thickness of the film.
Quarterwave stacks and gradient index optical films have limitations that make them unsuitable for certain applications. Non-quarterwave stacks, also known as enhanced thin films (ETFs), typically comprise many alternating layers of at least two different refractive index materials at various thicknesses ranging from a few to several thousand angstroms. Non-quarterwave ETFs provide higher performance in applications such as sharp edged band filters, broadband antireflection coatings, and tristimulus filters, :for example. Depositing non-quarterwave ETFs with a hundred layers, for example, requires individual layer thickness tolerances that are difficult to meet with conventional monitoring techniques. Therefore, a new method of monitoring and controlling the deposition of multiple layer thin films is needed to achieve greater deposition accuracy and stability resulting in improved thin film performance.